Method inconstruction of subaqueous tunnels



1932- F. DONALDSON METHOD IN CONSTRUCTION OF SUBAQUEOUS TUNNELS Filed Oct. 3, 1931 [N VENTOR 7% M A I yORNEY Patented Oct. 18, 1932 warren s'ra'rss PATENT orrica FRANCIS DONALDSON, OF YONKERS, NEW YCRK, AS SIGNOR TO MASON & HANGER COMPANY, INC., OF NEW YORK, N. Y.,

A CORPORATION OF WEST VIRGINIA METHOD IN CONSTRUCTION OF SUBAQUEOUS TUNNELS Application filed October 3, 1931.

In the construction of subaqueous tunnels in sand and gravel or similar porous materials, two factors add to the difficulty and the cost of the undertaking. The first of these is that good practice and the laws of many States require the working hours of compressed-air-workers to be shortened as the pressure increases, so that the cost of work necessarily increases with the depth. The second is due to the difference in the external hydrostatic pressure at the top and the bottom of the tunnel face, whereas the internal pressure ofthe compressed air is constant over the entire face. As a result of this second factor, if air pressure enough is maintained to keep the bottom of the tunnel dry, there is serious danger in blowing off the roof. Conversely, if pressure is reduced so that only the top of the heading is kept dry, water and sand enter the tunnel at the bottom.

Up to the present invention no method has been devised for lessening the first-mentioned difliculty.

As to the second factor, wherever the depth of cover between the roof of the tunnel and the bottom of the overlying water is light, the danger of blowing off the roof is overcome by blanketing the bottom of the body of water, hereinafter referred to as the river, by means of dumped clay or other heavy impervious material.

My invention, as described hereinafter, is a method of lessening these two difiiculties. It involves sinking pump wells adjacent to the line of the tunnel and as close as practicable to the bank of the river, and operating these wells so as not only to lower the ground water level in the porous plain adjacent to the river but also to establish a down-draft through the river bottom, which, owing to the solids in suspension in the river water, will in turn establish on the bottom what I term a matte, which is practically watertight. The matte thus created is analogous to the schmutzdecke established in municipal sand fillers, which after a few days operation become so densethat the bed must be abandon-ed and a new one used while the filtered sand is washed and strained. In case the river or body of water does not contain sufficient Serial No. 566,687.

silt or fine organic matter in suspension to establish such a matte, this material may be provided at a comparatively slight expense by dumping liquefied silt or clayover and in the vicinity of the line of the. tunnel. As a result of long continued pumping, as above described, the ground water level in the plain adjacent to the river may be lowered from 15 to 30 ft. The matte may be thought of as acting more or less as a rubber membrane to prevent the hydrostatic head of the water in the river from acting in the ground through which the tunnel is being driven, and the required air pressure to keep the tunnel dry is measured by the hydrostatic head of the water in the plain adjacent to the river.

In further reference to the saving of cost by reduced air pressure, it may be stated that the laws of New York and Massachusetts provide that a compressed-air-worker may work 7 hours per day where the air pressure is below- 18 pounds above the atmosphere, equivalent to a head of water of40 ft; 6

' hours per day from 18 to 26 lbs. 4 hours from 26 to 33 lbs; 3 hours from 33 to 38 lbs; and so on. It is customary to increase the daily rate of pay fifty cents with each increase in air pressure. The savings to be efiected are, therefore, important.

The accompanying drawing is schematic. It indicates a tunnel 2 being driven beneath a river 3 with the aid of a shield 4. The

pump wells 5 are shown discharging their water to the river. As many wells are driven as are found necessary; one may be sufficient. In the particular tunneling operation in which the process is being employ-ed, highly satisfactory results have been obtained with wells at one side. of the river, but obviously still greater advantage may be realized by pumping from wells at both sides of the river. Wells might even be'sunk through the bed of the river. While it is desirable to have the wells as near the river as possible, it may not be practicable, due to local conditions, to place them very close. In the tunneling referred to the nearest well is about one thousand feet from the river bank.

Pumping is maintained at a rate to lower drawing, the line A represents the original ground water level, and thecurve B represents the lowered ground water level. Whereas the original hydrostatic head at any depth beneath the river would be measured by the distance to the surface of the river, the long continued pumping causes the effect of the curve B to extend across the river, so that the distance C represents the head saved in the part of the wgrk beneath the river. What is done, as previously stated, is to create a clowndraft through the river bottom, thereby producing the matte 6, the material for which may be supplied either by silt present in the water or by similar material dumped into the river. The pumping is continued after the matte or schmutzdecke has been established and while the tunnel is being driven beneath the body of water.

The result is to reduce the air pressure necessary in the tunnel, the amount of heavy blanketing that may be required, and the cost of the tunnel.

The walls should be drilled deep, or deep enough to get the volume of water required. The depth will naturally vary with the circumstances. It should be sufiicient to get below the zone of escaping air.

I claim:

1. In the construction of a subaqueous tunnel driven under compressed air, the method which comprises continually pumping water from the ground in the vicinity of the operation at a rate suflicient to create a, downdraft through the river bottom, and eventually establishing a substantially watertight matte, the material for which may be supplied as herein stated, and thereafter continuing the pumping while the tunnel is driven beneath the body of water, thereby reducing the hydrostatic pressure on the tunnel and the amount of air pressure required within the shield to enable the tunneling operation to be carried on while the tunnel is being driven beneath the body of water.

2. The method which comprises establishing a substantially wat-ertight matte on the bottom of a body of water beneath which a tunnel is driven under compressed air, and continually pumping water from deep wells in the vicinity of the tunneling operation, thereby not only lowering the ground water level adjacent to the body of water but also reducing the hydrostatic head on the tunnel as'it is driven beneath the body of water and thus reducing the amount of air pressure required within the shield when it is below the water body.

3. The method which consists in driving a tunnel under compressed air beneath a body of water, pumping water from the ground in the vicinity of the operation from a sufficient depth and at such rate as to create a downdraft through the river bottom, thereby establishing a substantially water-tight matte on of water.

FRANCIS DONALDSON. 

